The present invention generally relates to the mitigation of multipath interference in spread spectrum ranging systems, and even more particularly relates to methods and apparatuses for mitigating multipath interference in Global Positioning System receivers.
Spread spectrum positioning systems, such as the Global Positioning System (GPS) operated by the United States government or the Federation of Russia""s Global Orbiting Navigating Satellite System (GLONASS), are well known. Receivers have been developed that make use of positioning system signals for navigational and other applications. Navigational receivers, for example, serve land, sea and air transportation applications. Land applications include uses by ground vehicles and hand-carried uses by individuals. Other types of applications include surveying, geodetic control or plate tectonic studies. Yet other related positioning system applications serve telecommunication facilities, laboratories, astronomical observatories and other such facilities requiring precise timing or accurate frequencies.
The accuracy of the results obtained by a positioning system receiver is dependent on the quality of the received signal. The received signal, however, is often distorted by the presence of multipath interference. The environment surrounding the receiver determines the existence and characteristics of any received multipath signals. Hence, significant error can be introduced even in a differential GPS system. When multipath interference is present, the positioning and timing solutions calculated by the receiver are less accurate and the usefulness of the system is diminished.
In a GPS-type positioning system, multipath interference results in a distortion of the received pseudorandom noise (PRN) signal, which in turn leads to a distortion of the correlation function. The correlation function is the function obtained from the correlation of an internal, receiver-generated version of the PRN code with a received satellite-generated signal. Multipath-induced distortion causes a conventional code tracking loop in the receiver, for example a code tracking loop using an early-minus-late discriminant, to incorrectly identify the correlation function peak. The multipath-induced distortion causes the location of the correlation function peak to be shifted. Since pseudorange (PR) measurements are derived from the time associated with the correlation peak, a shifted or an incorrectly identified correlation peak leads to an incorrect position and time solution. The incorrect solutions arise because the multipath interference does not cause all of the received PRN signals to be shifted in exactly the same manner.
The multipath interference problem is well known in the art and various responses have been devised to deal with it. One approach has been via antenna design innovations, such as choke ring ground planes. A related approach involves careful selection of the receiver antenna site. Such approaches, however, are often impractical. In many applications, for example, the receiver and antenna must be capable of mobile operation in uncontrolled environments.
Multipath interference has also been addressed by attempting to better locate the correlation function peak or the peak of a multipath-free correlation function. One attempt involved modification of the early-minus-late (EmL) discriminant of the code tracking loop. Since the absolute values of the slopes on each side of the correlation peak are not equivalent when multipath interference is present, the EmL discriminate does not track the peak accurately.
The EmL discriminant was modified to take the different slopes into account. The modified approach is often called the Early-Late-Slope (ELS) technique. The ELS technique uses two correlators on each side of the peak to estimate the slopes on each side. The two lines formed by the pairs of correlators intersect at a point. The point of intersection is used as the estimate of the peak of the correlation function. A failing of the ELS technique, however, is that it does not take into account the rounding of the correlation function by the receiver""s front-end filtering.
Yet another multipath mitigation technique involves measurement of the correlation function and the estimation therefrom of the underlying direct and multipath signal parameters (amplitude, phase and time delay). The estimated multipath signals are separated from the correlation function and the remaining function is taken as an estimate of the direct signal.
The method of reducing the correlation function into estimated direct and multipath components is problematic for several reasons. First, for example, the approach requires implementation of a model predicting the multipath environment that the receiver will experience. For instance, the model may assume that a specified maximum number of multipath reflections will be received with every direct signal. The model, for example, may be constructed to solve for a direct signal and two related multipath signals. Such an approach may provide acceptable performance when the circumstances expected by the model match the environment actually experienced by the receiver. When operating conditions conflict with the model""s assumptions, however, the performance of the system is degraded.
Further, implementation of such an approach requires significant hardware and software overhead. A plurality of correlators, sometimes ten or more, are required for a single tracking channel. A typical receiver, such as one employing an EmL technique, would use only two or three correlators per tracking channel. The increased number of correlators requires additional software processing and additional hardware.
Consequently, there is a need for improved methods and apparatuses for mitigating for the effects of multipath interference. Methods and apparatuses are needed which compensate for the multipath-induced shift of the correlation function peak. In addition, improved correlation function peak tracking is needed. Such improvements would desirably include an approach for measuring multipath bias that requires only a small number of correlators per channel.
It is an object of the present invention to provide a spread spectrum positioning system receiver capable of determining present position with enhanced precision.
It is a feature of the present invention to mitigate the presence of multipath interference in a received spread spectrum pseudo-random coded ranging signal.
It is an advantage of the present invention to provide a multipath mitigation approach requiring only a small number of correlators per channel.
The present invention is an apparatus and method for mitigating multipath interference in a spread spectrum ranging signal received by a positioning system receiver. The invention is designed to satisfy the aforementioned needs, provide the previously stated objects, include the above-listed features and achieve the articulated advantages. The present invention is carried out, using a limited number of correlators, in a manner that does not require establishment of assumptions concerning the receiver""s future operational environment.
Accordingly, the present invention is a positioning system receiver that mitigates multipath-induced errors by improved peak tracking, by peak shift compensation, or by a combination of both techniques, to provide more accurate timing and positioning solutions.